1. Field of the Invention
This invention relates to simulated leather sheet material and to a method of preparing such simulated leather sheet material.
2. Description of the Prior Art
Natural leather, appropriately finished, is valued for its durability and aesthetic characteristics for a plurality of uses. Due to the scarcity of leather and the increased cost of processing leather for particular applications, economics have dictated that synthetic materials be substituted in certain applications where leather goods had been used. Such synthetic materials have been proposed and used in the areas of shoe uppers, upholstery, clothing, luggage making, book binding and similar applications. Because these various applications require differing physical, chemical, and aesthetic qualities, different processes using different materials must be used to obtain an acceptable product which is comparable to natural leather; although in most instances these synthetics are readily distinguishable from natural leather.
Natural leather from animal hides is composed of two surfaces: one surface defining the grain layer, which in most instances is the most aesthetically desirable and the opposing surface defining the split layer. The grain layer is the epidermis of the animal and is very smooth whereas the split layer in most instances is rough and fibrous.
One method of preparing a synthetic as a substitute for leather involves impregnating and/or coating of porous material, for example, cloth, with a polyurethane, vinyl or a similar material. Polyurethanes have met with wide acceptance as a coating or impregnating composition due to their capability of wide variation in chemical and physical properties particularly their flexibility and chemical resistance.
Objectives in preparing the synthetic substitutes for leather are that they provide: (1) sheets especially suitable for leather-like and upholstery uses; (2) sheets of uniform width as commonly used in the textile industry (unlike natural products which sustain substantial weight and area losses in cutting and finishing); (3) end use versatility, for example, under a variety of exposure conditions where certain chemical treatments will assist maintenance and useful lifetime of properties; and most importantly, (4) a product with the strength, hand, drape and softness comparable to natural leather.
Further, a simulated leather sheet material when used for shoe uppers should be characterized by a leather appearance, with no undesirable fabric show through, good water vapor permeation into the uncoated side of the upper, and a leather grain break (minimal gross wrinkling). "Leather-like grain break", as recognized in leather and upholstery industries, is manifested in the behavior of well finished leather when folded or crumpled. The leather fold is characterized by a smooth curved contour, frequently with numerous fine wrinkles in the compressed region of the fold area. This is contrasted with sharp creases or gross wrinkles formed when papers or films are folded; this kind of undesirable appearance is known as "pin wrinkling."
The "hand" of leather is highly distinctive and synthetics normally have a rubbery feel which is contrasted with leather.
Polyurethane polymers as coatings or impregnants for fabric to provide substitutes for leather have long been recognized. For example, polyurethanes can be made which are highly resistant to solvents and abrasion, conferring dry cleanability and outstanding durability to coated fabrics. The basic chemistry of polyurethanes, involving reactions between the isocyanate groups and molecules with multiply reactive hydrogen, such as polyols and polyamines, afford great versatility and variability in final chemical and physical properties by the selection of intermediates to achieve processibility and the desired balance of end use performance requirements.
There are various methods for applying polyurethane solutions or other post curable liquid polymers to porous substrates which are well known to those skilled in the art. An article in Journal of Coated Fabrics, Vol. 7 (July 1977), pages 43 through 57 describe some of the commercial coating systems, e.g. reverse roll coating, pan fed coater, gravure and the like. Brushing and spraying may also be used to coat polyurethanes on porous substrates. These polyurethane solutions, after impregnation or coating on the porous substrate, are dried or cured by a method such as heated air, infrared radiation and the like. Characteristic of these processes is the deposition of a polymer and a film like layer which tends to produce a coated fabric which folds in undesirable sharp creases rather than leather-like grain break. Other methods of combining polymeric solutions and particularly polyurethane solutions with porous substrates are exemplified by U.S. Pat. No. 3,208,875 and U.S. Pat. No. 3,100,721.
An improved process for impregnating fabrics is disclosed in U.S. Pat. No. 4,171,391 and an even further improvement is disclosed in U.S. Pat. application Ser. No. 188,329, filed the same day as this application, by John McCartney entitled "Impregnated Non-Woven Sheet Material" both the patent and patent application are incorporated herein by reference and made a part hereof. Both the cited application and cited patent include certain steps which are necessary in forming simulated leather sheet material in accordance with the invention.
In accordance with the present invention, a simulated leather sheet material is formed which has the appearance and properties of natural leather and further has certain physical similarities therewith.